Application Of CO₂ Treatment Technology In Steam-Cured Cement-Based Materials

The rapid development of the construction industry has led to a sharply increasing demand for steam-cured prefabricated concrete and steel bars.It has also caused many problems which should be concerned,such as excessive emission of CO₂,thermal damage of steam-cured concrete and rational utilization of steel slag.Based on the carbonation reaction between CO₂ and cement/steel slag,this thesis mainly discussed the application of CO₂ gas in the surface treatment of steam-cured cement-based materials,and the rational utilization of CO₂ gas and steel slag in steam-cured cement-based materials.The feasibility of the application of steel slag aggregate treated with CO₂ in steam-cured mortar was discussed,aimed at providing reliable data and theoretical support for the utilization of CO₂ and steel slag in steam-cured materials.The specific research results are as follows:（1）The effects of steam-cured temperature,CO₂ treatment methods and subsequent curing methods on the properties of mortar were studied.CO₂ surface treatment could effectively improve the mechanical properties and impermeability of steam-cured mortar,especially at the early age（3d）.In addition,compared with ordinary CO₂ treatment,pressurized CO₂treatment did better on the performance of mortar.After CO₂ surface treatment,the performance of 45℃steam-cured mortar was obviously better than that of 60℃.The combined curing methods of carbonation could reduce the steam curing temperature and fix CO₂,and obtained prefabricated components with the same performance as ordinary high-temperature steam curing.（2）The influence mechanism of combined curing of steam curing and CO₂ treatment on mortar was analyzed.Due to thermal damage,steam-cured mortar had defects such as surface cracks and pores,which could provide the way for CO₂ to infiltrate in.During the CO₂ surface treatment progress,CO₂ reacted with unhydrated cement clinker and its hydration products to form Ca CO₃,C-S-H gel with lower Ca/Si ratio and amorphous silica gel.In this process,the p H value of mortar decreased,but it could be increased by subsequent saturated lime water immersion and standard curing.The crystallinity of Ca CO₃ and the Ca/Si ratio of C-S-H gel also increased.At the same time,the pore structure in the surface of mortar was further improved.（3）CO₂ was used to deal with steel slag fine aggregate.The autoclaved pulverization rate of steel slag fine aggregate decreased.The longer the carbonation duration,the more sufficient the carbonation of steel slag.However,the carbonation rate decreased with carbonated time.The reason was that the carbonation rate was fast at the initial stage of carbonation and the easy-carbonated components were involved in the carbonation reaction.The generated carbonation products would block the pores of the specimen and hinder the subsequent carbonation reaction.The strengthening effect of carbonation on steel slag aggregate further improved the mechanical properties and impermeability of formed steam-cured mortar,and effectively alleviated the problem of volume stability of steel slag mortar.（4）Comparing the effects of optimal CO₂,pressurized CO₂and carbonated water treatments on the properties of steel slag fine aggregate.The effects of the two gas carbonation treatment methods were similar,but slightly different from that of carbonated water treatment.Although the total amount of fixed CO₂ of steel slag treated with gas carbonation was more,the effect of carbonated water treatment on the performance of steel slag was more significant.This might be the result of the simultaneous consumption of f-Ca O by carbonation and hydration.（5）The artificial steel slag aggregate was made of steel slag powder,cement and fly ash.After CO₂ treatment,stable Ca CO₃ crystals were formed,which made the microstructure of the artificial steel slag aggregate more compact,and the mechanical properties were significantly improved.The crushing value of it was reduced to less than 30%.The interface between the artificial aggregate and the hydrated cement slurry was well bonded,and the impermeability of the mortar was significantly improved.（6）Comparing the effects of ordinary CO₂,pressurized CO₂and carbonated water treatment on the properties of artificial aggregate,the effects of two kinds of gas carbonation treatment were significantly better than carbonated water treatment.For the artificial steel slag aggregate treated with carbonated water,carbonation and hydration reactions occurred at the same time,and a large number of Ca CO₃ crystals were produced,which was also accompanied by the dissolution and loss of hydration/carbonation products of the aggregate.Therefore,the performance of the aggregate treated by carbonated water was lower than that of the aggregate carbonized by CO₂ gas.